Understanding SFP
in Air Movement
Geoff Edwards, Technical Sales Engineer at Axair Fans briefly discusses the challenge of specific fan powers
for fan coil units.
ans represent an enormous
potential for energy savings
and the reduction of CO2
emissions, equating to 9.3%
of energy in the UK. This is
a significant contribution when looking
at lighting as a comparison which uses
about 10% of the whole EU’s electricity
consumption. Statistics show that in colder
climates the energy consumption of fans
in modern commercial buildings can
be as high as 20% of the building’s total
energy use and analysts predict that unless
building regulations are enforced to limit
fan power, this fan energy consumption
could potentially double over the next 20
years.
F
Seasonality highly effects the energy
consumption used by fans, for example
the energy used by a supply fan ends up as
internal heat gain in a building, whilst the
exhaust fan energy is lost to the outside.
This means a proportion of the fan energy
is transferred to the inside of a as heat
which is useful in the colder months when
heating the desired effect of ventilation
systems but causes a cooling requirement
load on the ventilation system during the
cooling system.
The term used to measure the amount
of energy used within a heating and
ventilation system is known as Specific Fan
Power (SFP). This energy is used to drive
fans relative to the amount of air that is
circulated. It is constant but changes with
both air flow rate and fan pressure rise.
The energy efficiency of fan coils is heavily
influenced by the SFP, the lower the SFP
the more efficient the fan coil. In the UK,
where fan coil units (FCU’s) are popular,
regulations set separate limits on SFP for
buildings and SFP for Fan coil systems.
FCU’s with 4 pole motors AC motors can
achieve SFP of 0.5~0.8 Watts per litre per
second (W/(l/s) depending on capacity,
while FCU’s with EC motors can achieve
0.15~0.4W/(l/s). The larger SFP of 0.5-0.8
from a single AC motor may not seem a
problem, but when multiplied by 500 or
1000 fans in a large building, this mounts
up to a large energy demand.
Over the past ten years, small air handling
units such as fan coil units have undergone
many changes to improve efficiency with
good reason and EC motors have brought
the biggest efficiency gains to the fan
coil market to bring energy consumption
well below half that of their traditional
AC counterparts. However power factor
may be an issue with EC fans in a fan coil
unit installation and active power factor
correction would be necessary to raise the
power factor of the installation to about
0.9.
Fans to produce air flow rate through the
unit are fitted in all fan coil units. They are
forward curved centrifugal fans, usually
double inlet although single inlet can
also be used. Fan motors can be 2 pole or
4 pole AC or EC, single phase induction,
capacitor start/run, up to approximately
200w capacity with sealed for life ball or
sleeve bearings. All motors fitted in fan coil
units have a limited torque characteristic
and any external air static pressure across
the unit, such as that created by ductwork
or grilles should be kept to a minimum.
In recent years fan coil unit installations
have become one of the most popular
types of air conditioning system
in both new build and refurbished
commercial premises. The basic reason
for installing fan coil systems is to provide
comfort conditioning at the specified
environmental conditions within the
space.
Whilst not an inherently complicated
system a number of problems can arise
with fan coil unit installations, such as
those related to noise, thermal output and
draughts, and care needs to be taken in
the manufacture, design, selection and
installation of the equipment to avoid
common pitfalls. An inadequate or badly
performing HVAC system can have adverse
effects on the occupants and studies have
been carried out that show a link between
substandard working environments and
productivity in the workplace.
Therefore it is evident that the need to cool
buildings will not go away and possibly
with the continued effects of global
warming, our demand for cooling will
increase. The challenge will be to continue
to deliver this cooling ability in an as
efficient way as possible when combining
the two considerations of installing
energy efficient products within an energy
efficient system with an intelligent control
system to integrate the two.
Axair Fans UK Limited offer an extensive
range of industrial fans for use within
original equipment manufacture such as
Fan Coil Units. With low noise, constant
pressure, constant volume and a range of
speed control options there’s an efficient
solution for current demands.
www.axair-fans.co.uk
Issue 27 PECM
15